PROJECT SUIVIMARY (See instructions): An important function of the endothelium lining the inner surface of blood vessels is to provide a selective barrier between blood and the surrounding tissues. During the development of acute lung injury (ALI) the endothelial barrier is weakened, leading to increased permeability. It is well known that the family of small ras homology (Rho) GTPases (RhoA, Rad, Cdc42) play a crucial role in the maintenance of endothelial barrier properties. The two best-characterized members of the Rho GTPases: Rho A and Rad, appear to regulate endothelial barrier function in an antagonistic manner. Thus, the activation of Rho A impairs barrier function whereas Rad appears to support barrier integrity. In addition, our recent studies have shown that during the development of ALI in the mouse lung the activities of RhoA and Rad are regulated in an opposing manner such that RhoA activity is increased and Rad activity is attenuated. Together these changes would favor barrier disruption. However, the mechanism by which this opposing regulation occurs unresolved and is the major focus of this project. We will evaluate the mechanisms by which uncoupled eNOS leads to modulation of RhoA/Rad balance through nitration-mediated modifications. We will also determine if preventing RhoA and Rad nitration is barrier protective in vitro and reduces lung injury in both G' and G^-mouse models of ALI, in vivo. It is anticipated that this Project using state-of-the-art cellular, molecular, biochemical, and physiological approaches that will not only increase our understanding of the mechanisms by which RhoA and Rad are regulated during both G(-)- and G(+) -induced ALI but will facilitate the development of new strategies and targets for the treatment of a disease that has not seen a significant drop in mortality in 40 years.